ISO Fits and Tolerances (ISO 286): The Complete Engineering Guide

ISO 286 Fits and Tolerances: A Practical Engineering Guide

ISO 286 defines the internationally adopted system of limits and fits used to manage dimensional variation and establish predictable assembly behavior between mating cylindrical features, such as holes and shafts. It couples two key ideas: the tolerance grade (IT grade), which controls the width of a tolerance band, and the tolerance position (letter code), which locates that band relative to the nominal (basic) size. With this system, design and manufacturing teams can communicate the clearance, transition, or interference targeted for a joint, choose appropriate manufacturing processes, and verify conformance by inspection.

This guide explains how ISO 286 works, when to use hole-basis or shaft-basis systems, how to select IT grades in relation to practical manufacturing processes, and how to evaluate real fit behavior with a worked example. It also highlights a critical but often overlooked consideration: post-processing layers like powder coating and plating can add 50–100 µm per surface, significantly altering fits, which makes masking of threads and bores essential.

How the ISO 286 System Works

Each size specification is a combination of a letter and a number, for example H7 for holes or g6 for shafts, appended to a nominal size such as 25 mm. The letter (uppercase for holes, lowercase for shafts) sets the fundamental deviation, which positions the tolerance zone above, on, or below the nominal size. The number (IT grade) sets the tolerance width in micrometres. Together, a hole and a shaft specification determine the possible clearance or interference of the assembled pair.

Key definitions used throughout ISO 286 include:

Basic (nominal) size: The target size (for example, 25 mm) to which deviations are applied.

Upper and lower deviations: The signed distances from the nominal size to the upper and lower limit respectively of the actual feature size. For holes they are written ES (upper) and EI (lower); for shafts, es (upper) and ei (lower).

Tolerance (IT grade): The numerical grade (IT5, IT6, IT7, etc.) that gives a tolerance band width. Higher numbers mean a wider tolerance, lower accuracy, and typically lower manufacturing cost.

Fundamental deviation: Sets the location of the tolerance band relative to the nominal. For instance, a hole with position H has EI = 0 µm (the lower limit lies at the nominal size), while a shaft with position h has es = 0 µm (its upper limit lies at nominal). Other letters shift the band above or below nominal to create specific types of fits with standard hole or shaft positions.

Clearance, Transition, and Interference Fits

ISO 286 classifies fit behavior in terms of the net difference between assembled hole and shaft. Three categories are important to design selection:

Clearance fits: The smallest hole is still larger than the largest shaft, so assembly always has positive clearance. This yields easy assembly and free running or sliding action. Examples and intended uses include:

• H7/g6 (a popular sliding/locational clearance fit for accurate guidance with minimal shake)

• H11/c11 (a coarse clearance fit for non-precision, easy-assembly joints)

Transition fits: Depending on actual sizes, the assembly may have a small clearance or a small interference. These are used where accurate location is needed without large assembly forces. A common case is H7/k6 or H7/js6 for light press or snug fits.

Interference fits: The largest shaft is larger than the smallest hole, so assembly always has negative clearance (overlap), producing a press or shrink fit. This is used for permanent or torque-transmitting joints. An example is H7/p6 for a reliable press fit.

Hole-Basis vs Shaft-Basis Systems

ISO 286 supports two ways to standardize fits depending on which feature is kept nominally tied to the basic size:

Hole-basis system: The hole uses position H (EI = 0), and different shaft letters (for example, g, h, k, p) create the desired fit. This is the most widely used system because holes are often produced by standard tooling (reamers, drills, boring tools), and it is easier to vary the shaft dimensions (by turning, grinding, or plating) to achieve different fits with the same hole size.

Shaft-basis system: The shaft uses position h (es = 0), while the hole letter is varied to achieve the fit. This can be useful when shafts are standardized (e.g., bar stock or shafts ground to a common size) and the hole must be adjusted for different fits. However, it is less common in typical production environments.

IT Grades and Practical Manufacturing Processes

The IT grade determines the tolerance width and should be matched to a feasible production process. Overly tight grades raise costs dramatically due to slower cutting speeds, finer tooling, stabilized environments, and increased inspection. Conversely, overly loose grades can cause functional issues like misalignment, vibration, or assembly problems. While achievable capability depends on machine condition, setup, and process control, the following mapping is widely accepted for design planning.

IT Grade	Typical Process Route	Approximate 25 mm band	Typical Use
IT6	Fine turning plus finish grinding	≈ 13 µm	High-precision fits, bearing journals
IT7	Precision boring, fine reaming, or honing	≈ 21 µm	Accurate location fits, quality sliding fits
IT9	General milling or reaming	≈ 52 µm	Locational clearance fits, non-critical alignment
IT11	Drilling (no ream), rough boring	≈ 130 µm	Coarse clearances, quick assembly

Notes on the estimates above:

• The bands are computed from standard tolerance units and rounded to practical values near mid-range 18–30 mm. Actual tables in ISO 286 must be consulted for exact values. At 25 mm, IT6 is about 13 µm and IT7 is about 21 µm, which are widely cited benchmarks.

• If surface finish or geometrical accuracy (roundness, cylindricity, straightness) is poor, it undermines the functionality of the fit, even if size meets IT grade limits. Grinding and honing can improve both size control and form.

Choosing Fit Types with Examples

H7/g6 – Locational Sliding Clearance Fit

A very common, tight clearance fit. Target applications include precise location with free sliding or very light push assembly. The H7 hole is easy to produce with quality reaming or boring, and g6 shafts can be ground or precision turned. You can expect small clearances that minimize play without risking seizure in clean, well-lubricated assemblies.

H7/p6 – Reliable Press (Interference) Fit

This interference fit yields secure torque transmission and location stability under load. It is used for mounting gears, pulleys, collars, and bearings where fretting must be minimized. The p6 shaft zone lies above nominal such that even the smallest shaft is still larger than the smallest hole. Assembly typically requires controlled pressing or thermal methods (heating the hub or cooling the shaft).

H11/c11 – Coarse Clearance Fit

Chosen for non-precision assemblies where speed and ease of assembly dominate and play is acceptable. This fit accommodates high production variability (for example, drilled holes and roughly turned shafts) and is suitable for temporary fixtures, jigs, guards, covers, and low-accuracy linkages.

Worked Example: 25 mm H7/g6 Fit

The goal is to determine the limits for a 25 mm nominal hole and shaft specified as H7/g6 and then compute the resulting clearance range. The steps below outline a practical design calculation aligned with ISO 286 conventions.

Step 1: Establish tolerance bands (IT grades) near 25 mm

For basic sizes in the 18–30 mm range, the standard tolerance unit leads to approximate IT bands as follows:

• IT7 ≈ 21 µm

• IT6 ≈ 13 µm

These are the widely used values for this size range and are adequate for engineering calculations. For contractual or safety-critical work, always reference the ISO 286 tables directly.

Step 2: Hole limits for H7

For a hole with position H, the lower deviation EI = 0 µm by definition. The upper deviation ES equals the IT grade width. Therefore:

• Hole lower limit: 25.000 mm

• Hole upper limit: 25.000 mm + 0.021 mm = 25.021 mm

Step 3: Shaft limits for g6

The g position is a shaft tolerance zone located slightly below the nominal size (negative deviations). Using standard tabulated ISO 286 fundamental deviations for letter g in this size range, a widely used set of values is:

• Shaft upper deviation es ≈ −0.004 mm

• Shaft tolerance (IT6) ≈ 0.013 mm

Therefore the shaft lower deviation is:

• ei = es − IT6 ≈ −0.004 mm − 0.013 mm = −0.017 mm

That yields:

• Shaft upper limit: 25.000 mm − 0.004 mm = 24.996 mm

• Shaft lower limit: 25.000 mm − 0.017 mm = 24.983 mm

Step 4: Clearance range

The minimum clearance occurs with the smallest hole and largest shaft:

• Cmin = 25.000 mm − 24.996 mm = 0.004 mm = 4 µm

The maximum clearance occurs with the largest hole and smallest shaft:

• Cmax = 25.021 mm − 24.983 mm = 0.038 mm = 38 µm

Interpretation

At 25 mm nominal, H7/g6 produces a tight, predictable clearance range (approximately 4–38 µm) ideal for accurate guidance with free sliding. In applications with contamination, thermal gradients, or limited lubrication, you may choose a slightly looser fit (for example, H8/f7) to avoid seizure risk.

Designing with Fits: Practical Considerations

Function first, then manufacturability: Define what the assembly must do. Is friction desired or harmful? Is torque transmission required? Is precise location more important than easy assembly? The answer determines clearance vs transition vs interference and the needed tightness of control.

Choose a basis: Default to hole-basis (H) because it is efficient to hold holes at nominal with standardized tooling, and you can tune the shaft letter to achieve the fit. Shaft-basis is useful when shafts are standardized and holes are flexible (for example, adjustable boring head in-tool assembly lines).

Match IT grades to processing capability: The process sets cost and feasibility boundaries. For instance, asking for IT6 on a drilled hole is impractical; it drives rework or scrapping. Similarly, specifying IT11 on a bearing seat risks vibration and early wear. Use the mapping to ensure a realistic route: IT6 by grinding, IT7 by precision boring/reaming, IT9 by milling or standard reaming, IT11 by drilling or rough boring.

Control form, not just size: Roundness, cylindricity, and surface texture matter. A “size-correct” shaft with lobing can bind in a hole. Similarly, bell-mouthed holes may yield exaggerated play after initial seating. Add geometric tolerances or process controls when function is sensitive to these issues.

Consider temperature and materials: Fits change with temperature. Interference that is secure at room temperature may relax at elevated temperatures if the hole material expands more. Conversely, assemblies done cold may seize hot. For dissimilar materials (aluminum hub, steel shaft), evaluate thermal expansion to avoid loss of preload or crack risk.

Account for coatings, plating, and paint: Secondary layers dramatically alter effective dimensions. Planning for them is integral to fit selection (see the dedicated section below).

Secondary Processes: Powder Coating and Plating Effects

Coatings add material to each surface they cover, which reduces clearances for external features and reduces diameters for internal features. Powder coating and many plating processes commonly add 50–100 µm per surface. That is not per diameter—per surface. The effective diameter change for a cylindrical feature is twice the layer thickness because the layer is applied to the full circumference.

Practical implications include:

• A 50 µm coating on a 25 mm shaft increases the diameter by approximately 100 µm. A fit like H7/g6 that had 4–38 µm clearance would flip to interference, risking seizure on assembly.

• The same 50 µm coating inside a hole reduces its diameter by approximately 100 µm. A hole finished for a clearance fit can become undersize, again turning into an interference situation.

• Most threaded assemblies are highly sensitive: a 50–100 µm per-surface deposit can lock up nuts and gall threads. For this reason, masking of threads and precision bores during coating is essential unless the feature was pre-machined oversize specifically to compensate for the final coating thickness.

Good design practice is to either:

• Exclude coatings from critical fits by specifying masking on drawings (for bores, bearing seats, datum surfaces, and threads), or

• Pre-compensate dimensions for the nominal coating buildup. For example, if a 25 mm H7/g6 sliding fit must be coated with 75 µm per surface powder coat on the shaft, pre-machine the shaft 0.150 mm smaller in diameter or relocate the tolerance zone to maintain the intended clearance after coating. This requires tight control of coating thickness variation and post-coating inspection.

Where coating thickness varies, consider using a post-coating sizing operation (for example, precision grinding of shafts or reaming/honing of bores) or select a less thickness-variable finishing process if the functional surface cannot be masked.

Verification and Metrology

Accurate fits demand matching metrology. Use calipers for rough checks only; for IT7 and tighter, employ micrometers, bore gauges, and ring/plug gauges. Confirm not only size but also roundness and surface finish. For interference fits, check runout and coaxiality to avoid assembly-induced stress concentrations or vibration.

Common Pitfalls and How to Avoid Them

Over-specifying tight grades: IT6 or better on large batches of general-purpose parts multiplies cost and schedule risk. Choose the least tight grade that still meets function.

Ignoring coatings and platings: The 50–100 µm per surface buildup is enough to ruin precision fits. Document masking, or pre-compensate with explicit basic sizes and tolerance positions accounting for layer thickness and variation.

Neglecting environment and maintenance: A design that functions in clean, lubricated conditions may seize in dusty, dry use. Add clearance margin or specify seals and lubrication accordingly.

Relying only on size tolerances: If alignment or rotation quality matter, add geometric controls (cylindricity, coaxiality) and surface finish requirements.

Additional Examples and Selection Guidance

H7/g6 (clearance): Use for precision sliding fits in jigs, fixtures, valve spools, dowel-like locators with removable shafts, and high-quality sliding machine elements. Provides minimal clearance with good location accuracy, suitable for frequent assembly/disassembly without tools.

H7/p6 (interference): Use where consistent press is needed but with moderate assembly forces and predictable retention: gears on shafts, collars, couplings. Evaluate thermal assembly options and verify hoop stresses in the hub, especially for brittle materials or thin-wall hubs. Consider H7/n6 or H7/m6 if slightly less interference is acceptable, or H7/r6 if more is needed.

H11/c11 (coarse clearance): Use where rapid production and easy fit are priorities; for example, hand-assembled guards, light brackets over standoffs, and non-locating sleeves. The wider tolerances tolerate drilling and rough turning without secondary finishing. Expect noticeable play and reduced positional accuracy.

From Requirement to Specification: A Short Workflow

1. Define function. Decide on clearance versus interference based on torque transmission, motion resistance, and assembly method.

2. Set a basis. Default to hole basis (H) unless there is a strong standardization reason for shaft basis.

3. Choose a letter for the mating feature to establish the type of fit (for example, g for tight clearance, k for transition, p for interference).

4. Pick the IT grade based on process capability and volume (for example, IT7 for precision bored holes, IT6 for ground shafts, IT9 for milled features, IT11 for drilled holes).

5. Check the resulting clearance/interference numerically at the intended size. For critical designs, simulate stack-ups across operating temperatures and coatings.

6. Document coatings, masking plans, and any post-coating sizing operations directly on the drawing and process router.

Summary

ISO 286 provides a powerful, standardized way to translate functional intent into manufacturable tolerances and predictable assembly behavior. By understanding how tolerance positions (letters) and IT grades (numbers) work together, engineers can select clearance, transition, or interference fits that deliver the right balance of location accuracy, running quality, and retention force. The hole-basis system, anchored by the widely used H position, simplifies production by keeping holes at nominal and tailoring the shaft to achieve the desired outcome. IT grades must be chosen with an eye toward real process capabilities: grinding for IT6, boring or fine reaming for IT7, milling or general reaming for IT9, and drilling or rough boring for IT11. A worked 25 mm H7/g6 example shows how to calculate limits and derive a practical clearance band of roughly 4–38 µm for a precise sliding fit.

Finally, do not overlook the impact of powder coating and plating. With typical build-ups of 50–100 µm per surface, unplanned coatings can swing a carefully engineered clearance into hard interference or render threaded features unusable. Proper masking of threads and precision bores, or explicit pre-compensation and post-coating sizing, is essential to preserve the intended ISO fit after finishing. By integrating these considerations early, designers and manufacturers can achieve robust, economical assemblies that work the first time and every time.